I have to disagree on this one (and similar warnings). There is no reason for being careful unless you are working with a server or somethng. The N900 is no server.

The worst thing that can happen is a bricked device that have to be reflashed. If you have a N900 without reflashing it, you are one out of two:
1. An expert that knows exactly what to do at all times.
2. WAY too careful.

Just my opinion